It's no fun having everyone discussing your breasts, especially in relation to cancer. So Angelina Jolie's decision to speak out about her double mastectomy has understandably won her much praise and many new fans. But in among the plaudits for a film star willing to risk such a personal revelation, there's a danger that something might be lost. Because the truth about breast cancer, as with so many other diseases, is that genes are not everything.

While one in eight British women will get breast cancer in their lives, only a small percentage of these diagnoses will be linked to known genetic mutations. About 40% of all cancers are thought to be related to lifestyle factors, but in the majority of cases the causes are complex and no one knows why cells go rogue.

My mother, like Jolie's, died of cancer in her 50s. When several other relatives were diagnosed with the disease within the next 10 years, I decided to have a genetic test. Fortunately I live in the UK, where the NHS offers free genetic screening, so I didn't have to make any difficult financial decisions. But after filling in several forms and having a long conversation with a very sympathetic genetic consultant, I was told my risk of getting breast cancer was no higher than average as the other cancers in my family had no relation to breast or ovarian.

This was a massive relief. Since my mother's death I'd been convinced that I would be next and had been very conscientious about checking my breasts regularly. I think the relief made me a bit complacent.

Two years ago, at 45, I discovered a lump in my breast which turned out to be malignant. By the time I found it the cancer had spread to my lymph nodes, which meant I needed more extensive surgery and more aggressive chemotherapy than if it had been discovered earlier. It also means that the risk of it returning is significantly higher, and my prognosis poorer.

I have subsequently had a genetic test after deciding that I would have a preventative mastectomy on the other breast if I had a faulty gene. But it turned out to be negative – so there's no suggestion that if I had been tested in the first place my cancer could have been avoided. In fact, it may well have made me even more careless about checking regularly.

At the time, the consultant made it clear that there's a lot that medical science doesn't know about the genetic causes of disease. "If your DNA is like Great Expectations," she said, "we're talking about a spelling mistake somewhere. Unfortunately we don't know where and we don't have an accurate spellchecker."

As scientific research continues, more genetic markers for all diseases will be found and tests will become better predictors. But I'm prepared to bet that there will never be a simple one-to-one relationship between genes and illness. Just like we'll never fully solve the nature v nurture debate.

So while Jolie's decision to go public is fantastic for raising awareness about breast cancer, there are potential pitfalls. The tests are extremely expensive, and an increase in demand for screening could add to the strain on NHS finances – although of course preventing cancer is much cheaper than treating it, even with the costly reconstructive surgery that Jolie had. More dangerous, perhaps, is the complacency or fatalism that a reliance on genes can foster. Experts still say that regular personal checks are the best way to catch breast cancer in its early stages. That's not something to lose sight of.